The half-marathon organisers described it as “the perfect opportunity to
achieve your personal best”.

They boasted of a course which was “flat and fast with amazing scenery”.

But when thousands of runners did, indeed, post their best performances, it was not the scenery or the lack of hills that could be thanked.

It was the fact that the course was short.

Officials at the Cardiff Half Marathon were forced to tell the runners that their times counted for nothing, as the course had been shortened for health and safety reasons.

Building work on a section of the route meant scaffolding was exposed, which officials had deemed a risk for the 15,000 participants in the event, many of whom had travelled from across the UK and abroad to take part.

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A last-minute detour had been created but no one thought to check the length of the adjusted course and the race went ahead.

The throng of amateur athletes - each of whom had paid £28 to take part - pushed themselves around the city centre course, from Cardiff Bay, along the Taff and past the Norman keep of the splendid castle, which dates back to the 11th Century, and back to the Bay for the finish.

However, a series of runners with accurate sat nav gadgets measured their times and found that instead of 13.1m, they appeared to have run slightly less.

They reported their findings to the organisers who hurriedly checked their figures and sent an apologetic email to the runners.

The email from Cardiff Half Marathon said: “Unfortunately for safety reasons there was a last-minute amendment to the Lloyds TSB Cardiff Half Marathon route.

“We are currently investigating the impact of this on the route length and timings for runners.

“The change in the course had to be undertaken as an obstruction was identified on the official route that resulted in a decision being taken at the last minute to ensure the safety of all runners.”

Mike McGeogh, 55, who has run over 70 half marathons around the world said he knew something was up when he saw his watch.

“I have been running these for about 30 years and I know very accurately what time I'm going to run them in,” he said.

“When I was at least a minute quicker than my usual time I suspected a problem.

“My reaction when I crossed the finish line was anger, and I'm still angry.”

The race is an annual event, organised by Barnardo’s, Cardiff City Council, Welsh Athletics and the Welsh Assembly Government.

It raised an estimated £100,000 for charity.

A spokeswoman for the organisers apologised to the runners and said they were investigating how it had happened.

She added: “Following an investigation, it was discovered that the course was 0.12m short.”